Several embodiments of methods, arrangements and plants relating to the above-mentioned technical field and with a function and nature that meets requirements are previously known.
Thus, different methods and arrangements are known for the separation of tall oil products (soap products) from a drawn off black-liquor containing boiling liquid, with very small fractions of tall oil products.
The boiling liquid is produced within a kraft mill digester where the abovementioned boiling liquid comprises a fibrous cellulose material in a mixture with the mentioned black liquid with tall oil products.
The boiling liquid shall primarily be brought to a mechanical separator unit, a sieve, in which solid fibrous cellulose material can be separated from a more viscous mixture of black liquor and tall oil products.
The thus separated fibrous cellulose material is brought to a plant for the production of paper pulp.
The viscous mixture of black liquor and tall oil products is brought, after addition of intermediate liquor to form mixed black liquor, to a separation tank that uses gravity separation, adapted to use gravity to separate tall oil products in the mixed black liquor from the mixed black liquor, said separation tank being adapted for being discontinuously emptied of collected and gravity-separated tall oil products, the tall oil products being positioned above and floating on a mixture of black liquor or mixed black liquor, and where the mentioned thus tall oil products which have been separated by gravity, directly or indirectly shall be brought to a centrifugal separator.
A previously known method and associated arrangement is shown in the attached drawings where FIG. 1 illustrates the principles for a part of the arrangement in a kraft mill with a chip digester and an attached mechanical separator unit, for the separation of a fibrous cellulose material (fibers) from the boiling liquid (black liquor and small amounts of tall oil products).
FIG. 2 illustrates the part of the known arrangement where a mixed black liquor is treated to allow separation the tall oil products, which comprise a small volume, from black liquor or mixed black liquor and other rest products.
FIG. 1 and FIG. 2 will be discussed in detail below.
Prior art also comprises a method comprising tanks for separating mixed black liquor, where the mixed black liquor is continuously supplied and where the tall oil products and the black liquor and mixed black liquor is, through gravity separation, continuously drawn off.
Prior art also comprises a method comprising tanks separating mixed black liquor, where the mixed black liquor is discontinuously supplied and where the tall oil products and the black liquor and mixed black liquor is, through gravity separation, discontinuously drawn off.
The drawing off of tall oil products somewhat mixed with the mixed black liquor is done from the top portion of the tank while the drawing off of mixed black liquor is carried out from the lower portion of the tank.
The current invention is based on the discontinuous separation of mixed black liquor and an arrangement adapted for that method.
Prior art also comprises the use of a centrifugal separator which can be supplied with gravity separated tall oil products comprising small amounts of black liquor or mixed black liquor, where the tall oil products (soap), by having different densities, can be efficiently separated from black liquor and mixed black liquor and their contaminations.
The article “Maximizing tall oil recovery”, Drew J, Chemical engineering progress 72 (5):64 (1976) describes a method for the separation of tall oil from boiling liquid.
US2012296066 describes a method for the separation of tall oil with the aid of a centrifugal separator.